Functions
2.8 Monitoring Functions
SIPROTEC, 7SJ61, Manual
C53000-G1140-C210-1, Release date 02.2008
137
2.8.1.4 Monitoring of the External Transformer Circuits
Interruptions or short-circuits in the secondary circuits of the current and voltage transformers, as well as faults
in the connections ( important for commissioning), are detected and reported by the device. The measured
quantities are cyclically checked in the background for this purpose, as long as no system fault is present.
Current Symmetry
During normal system operation, a certain symmetry among the input currents is expected. The monitoring of
the measured values in the device checks this balance. The smallest phase current is compared to the largest
phase current. Asymmetry is detected if | Imin | / | Imax | < BAL. FACTOR I as long as Imax / I
Nom
> BALANCE
I LIMIT / I
Nom
.
Thereby Imax is the largest of the three phase currents and Imin the smallest. The symmetry factor BAL.
FACTOR I (address 8105) represents the allowable asymmetry of the phase currents while the limit value
8105 (address 8104) is the lower limit of the operating range of this monitoring (see Figure 2-43). Both param-
eters can be set. The dropout ratio is about 97%.
This malfunction is reported as „Fail I balance“.
Figure 2-43 Current symmetry monitoring
Phase Sequence
To detect swapped phase connections in the current input circuits, the direction of rotation of the phase currents
is checked. Therefore the sequence of the zero crossings of the currents (having the same sign) is checked.
The default setting assumes a positive phase sequence.
Currents: I
A
leads I
B
leads I
C
Supervision of current rotation requires a maximum current of
|I
A
|, |I
B
|, I
C
| > 0,5 I
Nom
.
For abnormal phase sequence, the message „Fail Ph. Seq. I“ is issued.
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